1. Field of the Invention
The present invention relates generally to semiconductor integrated circuit devices, and more particularly, to structure of a large scale integrated circuit device comprising a general purpose unit and a specific unit or custom unit and used for a specific purpose.
2. Description of the Prior Art
A semiconductor integrated circuit device has been known which is adapted to perform a specific function in accordance with the requirements of users or the requirements of an equipment and an apparatus to be mounted. FIG. 4 shows diagrammatically structure of such a conventional semiconductor integrated circuit device. This semiconductor integrated circuit device is referred to as a mixed LSI hereinafter. In FIG. 4, a general purpose unit WU and a specific unit SU are formed on a semiconductor substrate 1. The general purpose unit WU is structured by an integrated circuit such as a general purpose memory and a microprocessor, and the circuit operation thereof has versatility which can be generally applied to many other equipment and apparatuses by performing a standard operation. The specific unit SU is structured by an auxiliary circuit for allowing coincidence between an operational function of the integrated circuit constituting the general purpose unit WU and an operational function required for the LSI, and the circuit operation thereof is specified every specification of the LSI. In addition, bonding pads BP are provided on the semiconductor substrate 1 to surround the general purpose unit WU and the specific unit SU. The bonding pads BP are connected to a lead frame (not shown) by wires such as gold lines when the LSI is packaged. In an operation of the conventional mixed LSI, the general purpose unit WU and the specific unit SU function by receiving and sending signals from and to the exterior through the bonding pads, the wires and the lead frame. As described above, the conventional mixed LSI has two-dimensional device structure. Thus, as a function performed by a single mixed LSI increases in accordance with the requirement of a higher degree of function, the area of a chip increases. As one method for controlling the increase in the area of the chip and increasing the function, it is considered that structure of the mixed LSI comprises three-dimensional stacked structure. For example, this consideration is illustrated in a three-dimensional device such as an SRAM (Static Random Access Memory) and a photosensor which is described in an article by Yoichi Akasaka entitled "Three-Dimensional IC Trends" PROCEEDINGS OF THE IEEE December 1986 pp. 1703- 1714. The SRAM has two-layer stacked structure in which a memory cell portion is formed in a first layer and a peripheral circuit portion such as a sense amplifier and a decoder is formed in a second layer. In addition, the photosensor has three-layer stacked structure in which a select logic circuit portion is formed in a first layer, a digitizer is formed in a second layer and a photodetector is formed in a third layer. Each of the three-dimensional devices is divided every device blocks which does not perform a significant function by itself and has a relatively low function and the divided device blocks are stacked.
However, if such stacked structure is applied to the mixed LSI, the manufacturing processes become complicated, so that the manufacturing period becomes long. More specifically, in order to manufacture the mixed LSI having stacked structure, many processes such as growth of a crystal layer, patterning and wiring are required every layer. Consequently, the number of processes is increased as compared with the conventional mixed LSI having two-dimensional structure, and techniques peculiar to the stacked structure such as a planar technique and an SOI (Silicon on Insulator) forming technique are required, so that the processes become complicated. As a result, the manufacturing period becomes long. Originally, such a mixed LSI tends to be a product which differs depending on users, to be many kinds and small amounts of ordered products. Thus, the date of delivery of the product is an important factor. Consequently, the manufacturing period is unfavorably extended in place of high integration density achieved by making the product to have stacked structure.